Method for forming pn junctions in indium antimonide with special application to infrared detection



Nov. 16, 1965 c. M. MESECKE 3,217,379

METHOD FOR FORMING PN JUNCTIONS IN INDIUM ANTIMONIDE WITH SPECIAL APPLICATION TO INFRARED DETECTION Original Filed Dec. 9, 1960 FIG. I.

INFRA RIED LIGHT VACUUM INVENTOR Curtis M. Mesecke BY W ATTORNEY United States Patent METHOD FOR FORMING PN JUNCTIONS IN IN- DIUM ANTIMONIDE WITH SPECEAL APPLICA- TION T1) INFRARED DETECTTQN Curtis M. Mesecke, Dallas, Tex, assignor to Texas Instruments Incorporated, Dallas, Tern, a corporation of Delaware Original application Dec. 9, 1960, Ser. No. 74,830, new Patent No. 3,139,599, dated June 30, 1964. Divided and this application Nov. 5, 1963, Set. No. 336,216

11 Claims. (Cl. 29-253) This application is a division of patent application Serial No, 74,830, filed Dec. 9, 1960, and now US. Patent No. 3,139,599, dated June 30, 1964.

The present invention relates to compound semiconductors, and, more particularly, relates to a technique for forming PN junctions in indium antimonide. preferred embodiment of the invention the resultant PN junction indium antimonide device is used for detecting infrared radiation.

In the prior art, indium antimonide infrared detector cells have been made by starting with single crystal material of N-ty-pe conductivity, and then doping the indium antimonide with a P-type conductivity-producing impurity to form a PN junction in the semiconductor material. Infrared detector cells made in this manner suffer from the disadvantage of requiring an optical filter to prevent the detection of energy at wavelengths in the l to 2.5 micron regions. Also, prior art infrared detectors are not able to eliminate noise satisfactorily, resulting in a poor signal to noise ratio.

It is, therefore, an object of the present invention to provide a sensitive infrared detector which has a selective spectral response, thereby eliminating the need for optical filters. This is accomplished because the required filtering action takes place in the infrared cell itself.

It is a further object of the present invention to provide a sensitive infrared detector in which the signal to noise ratio and the scattering effect are improved over prior art detectors.

It is a still further object of the present invention to provide a sensitive indium antimonide infrared detector having a selective spectral response in which the particular band of frequencies to which the device is to be made sensitive is preselected, and the concentrations of the doping impurities for the indium antimonide are determined according to the desired frequency response.

It is a still further object of the present invention to provide a simple, inexpensive and reliable method for producing infrared detectors having the improved characteristics set forth above.

It is a still further object of the present invention to provide an improved method for forming PN junctions in indium antimonide.

Other and further objects, advantages and characteristic features of the present invention will become readily apparent from the following detailed description of preferred embodiments of the invention when taken in conjunction with the appended drawings, like numerals indicating like parts, in which:

FIGURE 1 illustrates a PN junction indium antimonide semiconductor device produced in accordance with the technique of the present invention;

FIGURE 2 is a perspective view of the semiconductor device of FIGURE 1 shown mounted on one part of a two-part casing prior to vacuum sealing the semiconductor device inside the casing, the encased indium antimonide semiconductor device being used as an infrared detector;

FIGURE 3 is a longitudinal sectional view of the ice indium antimonide device as sealed in the two-part cas- FIGURE 4 illustrates schematically the apparatus used in the diffusion process for forming the PN junction.

Referring now to the drawings, FIGURE 1 illustrates the PN junction indium antimonide device which is designated generally by the numeral 10. The device 113 comprises a die of P-type indium antimonide into which an N-type layer 12 has been diffused, leaving a layer 11 of P-type material, with a PN junction disposed between the layers 11 and 12. The diffusant used to form the layer 12 is an alloy of one of the N-type impurities selected from the group consisting of sulfur, selenium, and tellurium alloyed with a carrier including indium and gallium. A contact 13 is alloyed to the N-type layer 12 and contains an N-type impurity such as sulfur, selenium or tellurium in an indium-gallium carrier. The same N-type impurity used to form the diffused layer 12 is used to form the contact 13. A lead wire 14 for the N-type layer 12 is connected to the contact 13. The lead wire 14 is preferably made of an alloy of gold and gallium. of gold, gallium and tantalum, or of pure gold. A tin or indium base plate 15 is alloyed to the P-type layer 11, and a lead 16, preferably of Kovar is connected to the plate 15. Kovar is the trademark for an alloy comprised of about 17 to 18% cobalt, 28 to 29% nickel and the balance iron.

When the indium antimonide PN junction semiconductor device of FIGURE 1 is used as an infrared detector, it is encased in a two-part glass housing, as is shown in FIGURES 2 and 3. The indium antimonide wafer 11? is mounted on a Kovar header 17 attached to cover the end of inner cylindrical wall 22 of a Dewar vacuum flask composed of inner cylindrical wall 22 and an outer cylindrical wall 23. The material of header 17 is selected to make a hermetic glass to metal seal with wall 22 as is well known in the art. The space enclosed within wall 22 constitutes a Coolant chamber 21. The walls 22 and 23 are joined by a Ushaped section of glass so that they define a chamber between them open at its upper end only. The lead wires 14 and 16 for the semiconductor device 10 are attached by spot welding or soldering to pins 35 and 36 of Kovar. Insulators 4t and 41 secure pins 35 and 36, respectively, to wall 22. An opening 37 is provided in the wall 23 of the flask and a fitting 24 is formed integrally with wall 23 to facilitate attachment of a hose leading to a suitable vacuum producing means.

The upper part of the glass casing comprises a cylindrical glass shell 30 open at one end and having a sapphire window 31 disposed in its closed end. The open end of the shell 30 is provided with an annular flange 32 which fits against an annular shoulder 25 on the rim of the wall 23. The shell 30 is sealed to the outer wall 23 of the Dewar flask to form a sealed chamber 45 for the indium antimonide device 11). After the two sections of the casing have been sealed together, chamber 45 is purged with an inert gas such as helium or argon, and then evacuated via fitting 24 to a vacuum of about 5 1O mm. of Hg.

During operation of the infrared detector, the chamber 21 of the double walled Dewar flask is filled with a suitable coolant, such as liquid N which maintains the device at a temperature of preferably around 77 K. The infrared radiation to be detected passes through the sapphire window 31 and impinges upon the indium antimonide wafer 11). Electric current is generated in the wafer 10 in accordance with the strength of the impinging radiation and passed to the leads 14 and 16. The leads 14 and 16 are connected via pins 35 and 36 to suitable amplifying and measuring circuits not shown.

The process for producing the PN iunction indium antimonide semiconductor device according to the principles of the present invention will now be described. The starting material is a Wafer cut along the (111) plane from a single crystal of P-type indium antimonide. The P-type conductivity is imparted to the indium antimonide by zone refining in which process one end of a bar of indium antimonide is converted predominantly to P-type conductivity. This material is regrown to be single crystal of P-type conductivity by a method similar to the well known Teal-Little method (described in U.S. Patent No. 2,683,676 granted to I. B. Little and G. K. Teal for producing crystals of germanium). To obtain the desired resistivity, the indium antimonide, if necessary, could be doped with such elements as cadmium, zinc or mercury as well as other P-type conductivity affecting impurities. The precise technique of obtaining P-type conductivity indium antimonide is of little consequence and forms no part of this invention The thickness of the single crystal is preferably around -40 mils, the resistivity preferably being from about .1 to about 25 ohm-cm. The wafer is etched in an etchant of saturated tartaric and nitric acid in the ratio by volume of 3 to 1 although this ratio is not critical. Since the crystal was cut along the (111) plane, the etch will at tack one side of the wafer at a faster rate than the other. This is because indium minutely protrudes at one surface of the crystal and antimony at the other. The indium or fast-etching side of the crystal is formed into an N- type layer 12 in the wafer by means of a solid state diffusion operation. The apparatus used to carry out the diffusion is illustrated in FIGURE 4. The apparatus comprises a specially designed Pyrex glass diffusion tube having a large chamber 51, a small chamber 52, and a narrow neck portion 53 disposed between the large chamber 51 and the small chamber 52. The end of the chamber 51 remote from the neck 53 is provided with a tube 54 and a valve 55. The valve 55 controls the gas pressure inside the chambers 51 and 52. When carrying out the diffusion operation according to one embodiment of the invention, the chamber is evacuated to a pressure of the order of 2 10 mm. Hg.

The diifusant for producing the N-type diffused layer 12 is an alloy containing either sulfur, selenium or tellurium alloyed with indium and gallium. The gallium is necessary for type conversion as well as to eliminate a surface reaction between the dope and the wafer. Also, the gallium probably in a form of indium-gallium antimonide effects the spectral distribution for both long and short wavelengths, since indium-gallium antimonide has a continuous range of energy gaps from that of gallium antimonide to that of indium antimonide. The purpose of the indium is to make the gallium easier to handle. In a preferred embodiment of the invention the diffusant consists of from about 0.05%-15% gallium, around l5%55% tellurium, selenium or sulfur, and the remainder indium mixed with approximately 50% InSb. A particular diffusant which has been employed success fully contains 50% tellurium. In FIGURE 4, the diffusant is illustrated as individual alloy pellets 56 located in the small chamber 52 of the diffusion tube 50. As referenced heretofore and hereinafter, percentage of material as given is percent by weight.

In carrying out the diffusion process, the dilfusant 56 is placed in the chamber 52, and the wafer oriented to maintain identity of the fast etched or indium side, is placed in the chamber 51 of the tube 50. The tube 50 is evacuated to the desired pressure, after which it is put into an oven (not shown) and heated to a temperature of from about 450 C. to about 510 C. The tube 50 is left in the oven for a time varying from 6 to 265 hours, depending upon whether sulfur, selenium or tellurium is used as the N-type conductivity-producing ingredient in the diffusant and also depending upon the bulk resistivity of the indium antimonide. Table I gives typical examples.

After the diffusion operation, the wafer (with the N-type layer 12 formed therein) is removed from the diffusion tube 50 and conditioned by removing the N- type layer from the Sb or slow-etched side by lapping. The wafer is then diced by ultra sonic cutting, sawing or similar method of area definition by similar means or by a photo-etch process. The dice are then suitably etched to remove damaged surface regions. Next, as illustrated in FIGURE 1, the contact plate 15 is attached to the P-type layer 11 of each die 70 and tab 13 is alloyed to the N-type layer 12. Tab 13 is composed of from 0.1 to 15% gallium, 0.05% to 5% S, Se or Te (matching the N-type impurity of layer 12) and the remainder indium or tin. Both plate 15 and tab 13 are alloyed to the die '70. The lead wires 14 and 16 are then connected to the contacts 13 and 15, respectively, by alloying. In the event the diffused die 70 is used as an infared detector cell, it is then mounted on plate 17 and sealed in the Dewar flask casing in the manner shown in FIGURES 2 and 3.

Although the invention has been shown and described with reference to particular embodiments, nevertheless, various changes and modifications obvious to those skilled in the art are deemed to be within the spirit, scope and contemplation of the invention as defined in the appended claims.

What is claimed is:

1. A method for forming a PN junction indium antimonide semiconductor device comprising diffusing an alloy of indium, gallium and an element selected from the group consisting of sulfur, selenium and tellurium into a wafer of P-type indium antimonide.

2. A method for forming a PN junction indium antimonide semiconductor device comprising diffusing an alloy of from about 0.05% to about 15% gallium, from about 15% to about 55% of an element selected from the group consisting of sulfur, selenium and tellurium, and the remainder indium into a water of P-type indium antimonide at a temperature of essentially between 450 C. and 510 C.

3. A method for forming a PN junction indium antimonide semiconductor device comprising diffusing an al- 10y of from about 0.05% to about 15% gallium, from about 15% to about 55% of an element selected from the group consisting of sulfur, selenium and tellurium, and the remainder indium into a wafer of P-type indium antimonide at a temperature sufficient for diffusion.

4. A method for forming a PN junction indium antimonide semiconductor device comprising heating a wafer of P-type indium antimonide and an alloy consisting of from about 0.05% to about 15% gallium, from about 15 to about 55 tellurium and the remainder indium to a temperature of essentially between 450 C. and 510 C. for about 6 to about 265 hours to diffuse said alloy into a region of said P-type indium antimonide.

5. A method for forming a PN junction indium antimonide infrared detector comprising diffusing an alloy of indium, gallium, and an element selected from the group consisting of sulfur, selenium, and tellurium into a water of P-type indium antimonide.

6. A method for forming a PN junction indium antimonide infrared detector comprising diffusing an alloy of from about 0.05 to about 15% gallium, from about 15 to about 55% of an element selected from the group consisting of sulfur, selenium, and tellurium and the remainder indium into a wafer of P-type indium antimonide at a temperature of essentially between 450 C. and 510 C.

7. A method for forming a PN junction indium antimonide infrared detector comprising difliusing an alloy of from about 0.05% to about 15 gallium, from about 15% to about 55 of an element selected from the group consisting of sulfur, selenium, and tellurium and the remainder indium into a wafer of P-type indium antimonide at a temperature sufficient for diffusion.

8. A method for forming a PN junction indium antimonide infrared detector comprising heating a wafer of P-type indium antimonide and an alloy consisting of from about 0.05% to about 15% gallium, from about 15 to about 55% of one of sulfur, selenium and tellurium and the remainder indium to a temperature of essentially between 450 C. and 510 C. for about 6 to about 265 hours to cause diffusion of said alloy into a region of said P-type indium antimonide.

9. A method for forming a PN junction indium antimonide infrared detector comprising heating a wafer of P-type indium antimonide having a resistivity of about 18 ohm-cm. and a thickness of about 30 mils in the presence of an alloy consisting of about 50% tellurium, about 5% gallium, and about 45% indium to a temperature of about a 0 510 C. for about 20 hours to cause said alloy to difiuse into said wafer of P-type indium antimonide and form an N-type layer in said indium antimonide.

10. A method according to claim 3 wherein the difiusion is carried out in a chamber evacuated to a pressure less than about 2 10- mm. Hg.

11. A method for forming a PN junction indium antimonide infrared detector comprising diffusing an alloy of indium, gallium, and an element selected from the group consisting of sulfur, selenium, and tellurium into a wafer of P-type indium antimonide to form an N-type layer in said wafer, attaching leads to said wafer, and mounting said wafer in a housing capable of being evacuated to a preselected pressure.

References Cited by the Examiner UNITED STATES PATENTS 2,780,759 2/ l957 Boyer 2925.3 X 2,798,989 7/1957 Welker 148-189 X 2,909,453 10/ 1959 Losco.

2,928,761 3/1960 Gremmelmaier 148-189- 2,933,662 4/1960 Boyer 2925.3 X 3,054,936 9/ 1962 Williams.

3,131,099 4/ 1964 Constantakes 148-189 RICHARD H. EANES, 111., Primary Examiner. 

11. A METHOD FOR FORMING A PN JUNCTION INDIUM ANTIMONIDE INFRARED DETECTOR COMPRISING DIFFUSING AN ALLOY OF INDIUM, GALLIUM, AND AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF SULFUR, SELENIUM, AND TELLURIUM INTO A WAFER OF P-TYPE INDIUM ANTIMONIDE TO FORM AN N-TYPE LAYER IN SAID WAFER, ATTACHING LEADS TO SAID WAFER, AND MOUNTING SAID WAFER IN A HOUSING CAPABLE OF BEING EVBACUATED TO A PRESELECTED PRESSURE. 